1. Field of the Invention
The invention concerns a process and a device for hydrodynamic sound pulse generation in a liquid, in particular for sound pulse generation at sea for marine geophysics measurements.
2. Description of Related Art
Although the invention is used preferably for sound pulse generation at sea for marine geophysics measurements, it is also suitable for other applications requiring hydrodynamic sound pulses, namely water depth measurement, using the echo sounding procedure, or even therapeutic and medical underwater treatments.
At the present time, sound pulses in liquids are generated through pulsed input of sound energy. This is achieved mostly by means of spark discharges or detonation of special explosives within the liquid. Generally speaking, however, piezoelectric and magnetostrictive sound generation cannot deliver strong sound pulses due to their limited amplitudes. Additional possibilities exist for aerodynamic sound pulse generation. A quantity of compressed air is introduced into the surrounding liquid. The increase in pressure in the liquid is due to the expansion of the compressed air and the resultant displacement of the liquid generates a sound wave. Sound pulse generation can be also generated through the collapse of cavitation bubbles.
Using known processes, however, a strong waved pressure signal which causes interference from multiple pulses is created. Moreover, the frequency range of the sound signal is, for the most part, limited to a few hertz and, therefore, is only marginally suited to examination of frequency-dependent dimensions such as, for example, the attenuation of sound waves at the bottom of the sea. Furthermore, the energy requirement for generating a desired sound intensity is high.
This invention meets the requirement of creating a process and a device by means of which strong sound pulses, sharply delimited in time, can be generated in the liquid, with higher bandwidths from the sound frequency spectrum and relatively low energy consumption.
In the process according to the invention: In a pipe open on one end, which surrounds a liquid column of a specified length that makes contact with the liquid at the open end of the pipe, the end of the liquid [column]opposite the open end of the pipe is subjected to sudden bursts of acceleration or deceleration in such a way that a sound pressure pulse is generated at this end of the liquid column; this pulse is transmitted through the liquid column to the open end of the pipe and from there it is radiated, on the one hand, into the liquid as a sound pulse, and is reflected, on the other hand, into the liquid column as a rarefaction wave.
According to the invention, the sound pulses are generated through the use of the well-known water hammer, which results from a very rapid change of flow inside a pipe within a time interval, which, according to Joukowski, must be shorter than twice the length of the pipe divided by the velocity of sound in the liquid. Under this condition, the liquid in the vicinity of the rapid change of flow becomes compressed, creating a sound wave, which causes, for example in pipes and other conduits of a pipe system, a knocking sound which can be very bothersome. In contrast, according to the invention, the water hammer is used for hydrodynamic sound pulse generation in a liquid, with the sound pressure pulse generated using the water-hammer principle radiated through the open end of a pipe into the liquid.
Simultaneously, a reflection occurs at this pipe end, and since it is an acoustically open pipe end, the pressure wave caused by the water hammer is reflected with a 180 degree phase angle shift, i.e., as a rarefaction wave, to the liquid column. Since this causes the pressure to fall sharply after arrival of the reflected rarefaction wave at the open end of the pipe, the rear flank of the emitted sound pulse drops steeply and this sharply delimits the radiated sound pulse in time. The duration of the transmitted sound pulse is thus determined by the length of the liquid column. Because of the rectangularity of the pressure signal obtained, the frequency spectrum includes, in addition to a primary frequency, its odd-numbered multiples as well as frequencies based on longitudinal waves in the pipe. On the whole, the result is a relatively high bandwidth for the radiated sound frequency spectrum. At the same time, the energy consumed for generating the sound pulse is relatively low since, by using the water hammer, pressures are generated that are many times greater than the excitation pressure.
To generate the water hammer at the end of the liquid column opposite the open end of the pipe, it is possible either to accelerate the liquid column in the direction opposite the open end of the pipe, and then abruptly decelerate it at the end opposite the open end of the pipe, or to permit an auxiliary mass, which has been previously accelerated in the direction of the open end of the pipe, to strike the end of the liquid column opposite the open end of the pipe abruptly. Both actions result in the compression desired according to the invention in the direction of the axis of the liquid column at the end opposite the open end of the pipe.
Furthermore, the invention offers the possibility of suppressing oscillation of the liquid in the liquid column after the radiation of the sound pulse, so that the radiated sound pulse is a single pulse without interference from multiple pulses. To facilitate this, in the refinement of the process according to the invention for generating the sound pulse, the end of the pipe opposite the open end is sealed by reverberation, which causes the rarefaction wave, originally created at the open end of the pipe through reflection, to be reflected again against the acoustically sealed pipe end without a phase angle shift as a rarefaction wave. The latter generates cavitation in the liquid column by which the subsequently reflected pressure wave at the open end of the pipe, following the reflection of this rarefaction wave, is attenuated, since the velocity of the sound is reduced by the cavitation.
The preferred device for the execution of the process according to the invention is, according to the invention, characterized by a pipe, open on one end, which surrounds a liquid column of a specified length which makes contact with the liquid on the open end of the pipe. The pipe is made of a material of low elasticity with a high mass moment of inertia and is bell-shaped at the open end. It is also characterized by a device which generates, in bursts, a compressive force for striking the end of the column opposite the open end of the pipe in the direction of its axis in order to cause sudden acceleration or deceleration of the liquid column, so that the sound pressure pulse, explained in connection with the description of the process, is generated. Since the pipe is made of a material with low elasticity, pressure changes due to cross-sectional changes in the pipe become negligible and, as a result, so become transverse waves transferred to the wall of the pipe, which have a different frequency spectrum and duration than the radiated sound pulse. Furthermore, since the pipe is made of a material with high mass moment of inertia, it is possible to prevent inducing a significant axial motion of the pipe, caused by a recoil pulse resulting from the generation of the compressive force, which would considerably weaken the water hammer pressure. It is preferable that the mass moment of inertia of the pipe be equal to at least 10 to 20 times the mass moment of inertia of the liquid column.
Moreover, since the open end of the pipe is shaped like a bell, a significant jump in impedance is prevented at the point of radiation of the sound pulse, and a good match of the sound waves in the pipe with the flat waves in the far field of the pipe opening is achieved through a gradual widening of the bell, which allows the pipe waves from the narrowest cross section of the pipe to spread out, without a jump, to an area whose diameter corresponds to, approximately, the wavelength of the primary frequency of the radiated sound pulses. In this case, the reflection to the open bell end becomes minimal, whereas the attenuation of the waves in the pipe, after radiation of the sound pulse, becomes maximal. It is preferable that the bell take the shape of a hyperbolic funnel.
It is obvious that, for the hydrodynamic generation of sound pulses according to the invention, the external appearance of the pipe has no particular significance. Therefore, a hole in an otherwise solid body can also fall under the designation of "pipe", as used here.
The length of the liquid column is greater than the diameter of the end of the liquid column opposite the open end of the pipe, preferably more than double this diameter. However, if very short pulses must be generated, the liquid column can be shorter than the diameter.
In a first embodiment of the device according to the invention, the device for generating the compressive force has a drive mechanism, for accelerating the liquid column in the direction opposite the open end of the pipe, and a piston which can be accelerated together with the liquid column and which works in conjunction with a stop for sudden deceleration of the end of the accelerated liquid column opposite the open end of the pipe. Thus, at the beginning of the acceleration of the liquid column, the piston is located in its home position, axially, at a distance from the stop. It can be equipped with a gripping device or the like to set the piston in the home position.
The drive mechanism can have an adjustable space at a lower pressure than that of the liquid in the liquid column next to the piston opposite the liquid column. For setting the piston in its home position, this space can initially be brought to relatively high pressure; then it can be deaerated to maintain the low pressure, so that the piston is accelerated toward the end of the pipe opposite the open end of the pipe under the pressure of the liquid along with the liquid confined by it.
It is also possible to equip the device with a tow link by means of which the pipe, with its bell pointing in the towing direction, is dragged through the liquid in order to accelerate the liquid and the piston in its home position. In this embodiment, there is a discharge area which opens into the liquid on the side of the piston opposite the open end of the pipe and out of which the liquid displaced by the piston can drain.
The piston can also work as a valve device in a two-way valve in conjunction with a valve seat which is located on the end of the liquid column opposite the open end of the pipe and whose valve opening opens into a discharge channel on the side of the valve seat opposite the open end of the pipe. This channel houses a flow drive mechanism to accelerate the liquid column in the direction opposite the open end of the pipe. The valve, in its open position, is located downstream from the valve seat and can be carried along by the accelerated liquid column all the way to impact on the valve seat. For resetting the piston in its home position, the flow drive mechanism can be reversible.
In a second, preferred embodiment of the device according to the invention the device for generating the compressive force has an acceleration device for accelerating an auxiliary mass and a device for allowing the accelerated auxiliary mass to strike the end of the liquid column opposite the open end of the pipe in the direction of the axis leading to the open end of the pipe.
This auxiliary mass can be a solid percussive body which works in conjunction with a piston by means of which the end of the liquid column opposite the open end of the pipe is axially confined and which is movable in the pipe all the way to impact a stop. After the percussive body is released, it strikes the end of the piston opposite the open end of the pipe in such a way that the piston is accelerated toward the open end of the pipe, thus accelerating the liquid column in front of it. In this embodiment, the mass of the percussive body is greater than the mass of the liquid column. Typically, it is 10 to 20 times the mass of the liquid column.
However, in this second embodiment, the auxiliary mass can also be a liquid mass. For example, the acceleration device can have a circular channel, wherein the interior of the pipe opens tangentially into the end of the pipe, and a flow drive mechanism for accelerating a liquid mass located in the circular channel and forming the auxiliary mass. The flow drive mechanism can be located downstream from the opening in the circular channel and a manipulable shut-off device can be provided, by means of which a cross section of the circular channel located downstream from the opening can be shut off after acceleration of the liquid mass. In this case, the flow direction in the circular channel is such that, after activation of the shut-off device, the circular flow is diverted tangentially into the pipe and thus strikes the end of the liquid column opposite the open end of the pipe.
In all embodiments of the device according to the invention, corresponding devices can be provided for cyclical repetition of the generation of sound pressure pulses.